This invention relates to a thermoplastic composite material. More particularly, this invention relates to a thermoplastic composite material which is particularly well suited for use as a bonding ply in a multilayer circuit board.
U.S. Pat. No. 4,849,284 which is assigned to the assignee hereof and incorporated herein by reference describes a ceramic filled fluoropolymer-based electrical substrate material sold by Rogers Corporation under the trademark RO-2800. This electrical substrate material preferably comprises polytetrafluoroethylene filled with silica along with a small amount of microfiberglass. In an important feature of this material, the ceramic filler (silica) is coated with a silane coating material which renders the surface of the ceramic hydrophobic and provides improved tensile strength, peel strength and dimensional stability.
The ceramic filled fluoropolymer-based electrical substrate material of U.S. Ser. No. 015,191 is well suited for forming rigid printed wiring board substrate materials and exhibits improved electrical performance over other printed wiring board materials. Also, the low coefficients of thermal expansion and compliant nature of this electrical substrate material results in improved surface mount reliability and plated through-hole reliability. As is known, individual sheets of this electrical substrate material may be stacked to form a multilayer circuit board. In fact, thin film formulations of the material disclosed in U.S. Pat. No. 4,849,284 (and sold by Rogers Corporation under the trademark RO-2810) may be used as a bonding ply to bond together the plurality of stacked substrate layers so as to form the multilayer circuit board.
While well suited for its intended purposes, it has now been determined that the fluoropolymeric composite material of U.S. Pat. No. 4,849,284 has certain drawbacks when used as a bonding ply to form a multilayer circuit board. For example, high temperature, high pressure lamination equipment is required for using RO-2810 bonding plys to laminate inner layers of RO-2800. This is a drawback as standard epoxy-glass lamination equipment operates at temperatures of less than 500.degree. F. and pressures of less than 300 psi. Thus, such standard and readily available equipment cannot be used to bond RO-2810 bonding plys thereby necessitating the purchase of more expensive equipment. Accordingly, there is a need for a thin sheet bonding material that can be used to laminate RO-2800 inner layers into multilayer structures at temperatures of less than 500.degree. F. and pressures of less than 300 psi so that standard epoxy-glass lamination equipment can be used. Of course, this material should possess all of the required properties of a circuit board substrate (chemical processing, temperature resistance, adhesion, etc.) and as many of the excellent electrical, mechanical and thermal properties (low Dk, low loss, low z-axis CTE) of RO-2800 as is possible.